Liquid crystal displays are finding an increasing variety of applications. Heretofore, liquid crystal displays have been limited in visibility under lighting conditions other than those for which they were designed. A more versatile liquid crystal display which is easily discernible under various lighting conditions would definitely enhance the usefulness of the display as well as broaden its field of practical utilization. The present invention is directed to compositions for providing such displays and particularly for displays visible in the dark.
The background art includes U.S. Pat. No. 3,844,637 to Masi et al, in which a composition is disclosed. In that composition, a fluorescent material, known as a luminophor is mixed with a nematic liquid crystal material. This luminophor is any suitable organic fluorescent material. As stated at column 2, lines 38-42 of the Masi et al patent, however, "The molecules of the luminophor attach and align themselves with the molecules of the liquid crystal and have little or no optical effect in the absence of an electric field." The composition disclosed in the Masi et al patent also requires the use of a pair of polarizers to orient the transmission of light so as to maximize the contrast ratio. From this, it is clear that the Masi et al composition is limited to use with polarized light and to polar fluorescent additives.
U.S. Pat. No. 3,960,753 to Larrabee also relates to a composition including in one state at least a liquid crystal phase. Fluorescent additives, among which is a europium chelate, are also included but the fluorescence would appear only marginal in the liquid crystal phase. Larrabee's fluorescent material is in the dissolved state and according to Larrabee, its "visible fluorescent intensity is dependent upon the phase or orientation of the liquid crystal solvent". (Column 1, lines 7-10) But from the Larrabee data and the discussion in "Fluorescent Switching by Means of Liquid Crystals", RCA Review, Vol. 34, June, 1973, pp. 329-335, on the same topic, it seems clear that Larrabee's systems are suitable only for thermo-optic devices--i.e., devices in which a change in temperature induces the phase change which results in visible display.
More akin to the present invention is the display and display composition disclosed and patented by White and Taylor. (U.S. Pat. No. 3,833,287 and Journal of Applied Physics, Vol. 45, No. 11, pp. 4718, November 1974) When the display of White and Taylor is in the cholesteric (OFF) state, the guest dye molecules and the matrix absorb light of all polarizations; in the ON state much less light is absorbed because of the homeotropic alignment in which the long axis of the dye molecule and the matrix are perpendicular to the electric field vector and therefore have minimal absorption. Here again, the visibility of the display depends on the orientation of the dye molecules of the liquid crystal. Also, the dyes of White and Taylor are not fluorescent. Nevertheless, White and Taylor do use a cholestericnematic phase change induced by an electric field as a basis for a display.
The present invention similarly uses an electrically-induced cholesteric-nematic phase transition, but in combination with a fluorescent compound of strong emissivity. As distinct from Masi, Larrabee, and White and Taylor, polarization of the absorption of the guest fluorescent molecule is not required.